1. Field of Invention
The invention relates generally to methods and apparatus for printing viscous material, such as solder paste, onto a substrate, such as a printed circuit board, and more particularly to a method and apparatus for calibrating the pressure or force of squeegee blades of a print head on a stencil, and for applying an accurate print pressure during production.
2. Discussion of Related Art
In a typical surface-mount circuit board manufacturing operation, a stencil printer is used to print solder paste onto a printed circuit board. A circuit board, broadly referred to as an electronic substrate, having a pattern of pads or some other conductive surface onto which solder paste will be deposited, is automatically fed into the stencil printer. One or more small holes or marks on the circuit board, called fiducials, is used to align the circuit board with the stencil or screen of the stencil printer prior to the printing of solder paste onto the circuit board. The fiducials serve as reference points when aligning a circuit board to the stencil. Once a circuit board has been aligned with the stencil in the printer, the circuit board is raised to the stencil by a substrate support, e.g., a table having pins or other work holders, and fixed with respect to the stencil. Solder paste is then dispensed by moving a wiper blade or squeegee across the stencil to force the solder paste through apertures formed in the stencil and onto the board. As the squeegee is moved across the stencil, the solder paste tends to roll in front of the blade, which desirably causes mixing and shearing of the solder paste so as to attain a desired viscosity to facilitate filling of the apertures in the screen or stencil. The solder paste is typically dispensed onto the stencil from a standard solder paste supply cartridge. After the print operation, the board is then released, lowered away from the stencil, and transported to another station within the printed circuit board fabrication line.
During a print cycle, as described above, the squeegee is moved across the stencil to force solder paste or any other viscous material through apertures formed in the stencil. FIG. 1 schematically illustrates the connection of a squeegee blade to a print head. As shown, a squeegee blade 40 is secured to a squeegee blade holder 64 in a position in which the squeegee blade may be disposed at an angle with respect to a stencil 18. The print head includes a first movable plate 54 and a second movable plate 56 that is connected to the first movable plate. The first movable plate 54 is secured to a frame (not shown) of the print head by two linear bearings indicated at 57 and driven by a lead screw 58, which is driven by a motor (not shown) provided in the print head. The arrangement is such that the lead screw threadably engages a lead nut 60 secured to the first movable plate to move the first and second movable plates 54, 56 along a path defined by the linear bearings 57. The second movable plate 56 includes a blade holder 64 to secure the squeegee blade 40 to the second movable plate. As shown, a compression spring 62 is disposed around the lead screw 58 to provide a resistance force between the first movable plate 54 and the second movable plate 56.
One issue with printing is calibrating the force upon which the squeegee blade 40 engages the stencil 18. Specifically, the spring constant of the compression spring 62 may vary. Also, when using a flexible (non-rigid) squeegee blade, the spring constant of the squeegee blade may effect the force upon which the blade engages the stencil 18. With reference to FIG. 1, a sensor 52 may be provided to a home position and/or a distance of the first movable plate 54 with respect to the frame of the print head. With the known calibration methods, e.g., replacing the squeegee blade 40 with a calibration gauge, the force of the blade against the stencil is determined by moving the first movable plate 54 a known distance, which is dependent upon the spring constant of the compression spring 62. Thus, if a customer has a completely rigid squeegee blade, the force of the blade against the stencil is somewhat accurate. It is difficult to determine the force of the squeegee blade against the stencil when the blade is flexible or when the spring constant of the compression spring is not to tolerance.